Conventional automatic transmissions transmit torque through what is commonly called a planetary gear system, where the planetary gears are in constant mesh and thus cannot clash. They also provide, through arrest of the various gear carrying members, means for drive ratio and direction selection. Brake bands under suitably hydraulic controls are sometimes used to effect these changes. The bands provide a means of holding gear carriers and usually act as apply force multipliers. As the governing equation is exponential in nature, a so-called double wrap band may have more than twice the holding power of an otherwise similar single wrap band, at the expense of increased stroke to full applied position.
Conventionally manufactured double wrap brake bands are formed from a relatively thick metal strap, and a metal forging or casting is butt-welded to the ends of the strap to provide apply and anchor bracket portions and a transverse bar portion for the band; the anchor and apply portions or ears being engaged by the struts and a hydraulic piston in the transmission, respectively, to actuate the band. Once the bracket is secured to the strap, the band and bracket may be heat treated and expanded to size, the bracket is transversely broached or milled to form the transverse bar portion and apply and anchor ears, openings are pierced through the bottom wall of the slots, the interior surface of the band is prepared for bonding and a friction lining applied thereto, openings are pierced through the lining, the interior surface is bored and the lining is grooved, and two circumferential slots are cut through the lining strap and bracket except for the transverse bar portion to result in three generally parallel radially expansible bands.
In copending U.S. patent application Ser. No. 310,340, an improved double wrap band and its method of formation is disclosed which obviates the disadvantages of the internal stresses created in previous bands formed by the above process that lead to distortion of the bands so they are out of round in the released position, thus creating undesirable friction drag between the band and clutch drum and uneven engagement of the band on the drum when actuated. The band of the above application utilizes a stamped preslotted thin steel band with transverse tie portions and a stamped bracket with longitudinal tie portions secured thereto. A preslotted friction lining is applied to the interior of the band and bracket, and the tie portions are punched out to form the three parallel band portions having improved roundness in the released position. This type of construction allows anchor configurations and spacings not possible in conventional construction bands.